1. Field of the Invention
The present invention relates to an optical branching device that is important as a basic device of optical communication systems, and particularly relates to a waveguide type, low-loss Y branch circuit of excellent productivity and stability and a method for manufacturing the same.
2. Description of the Related Art
With a backdrop of demand for various network services, such as the internet, video distribution, etc., FTTH (Fiber To The Home), with which optical fibers are laid to each household, is spreading rapidly. Among several network arrangements for FTTH, a PON (Passive Optical Network) arrangement is economical in that expensive transmission devices, to be installed in bureaus, and optical fibers, extending from bureaus to vicinities of user's homes, are shared by a plurality of users. Optical branching devices that distribute optical signals of a single optical fiber among a plurality of optical fibers are key devices of PON arrangements, and among such devices, waveguide branch circuits, having Y branch circuits arranged in tandem in multiple stages, are widely used due to being excellent in productivity and stability.
FIGS. 14A to 14E show a basic arrangement of a Y branch circuit. FIG. 14A is a plan view and FIGS. 14B to 14E are sections taken on section lines XIVB-XIVB′, XIVC-XIVC′, XIVD-XIVD′, and XIVE-XIVE′, respectively, of FIG. 14A.
The Y branch circuit shown in FIGS. 14A to 14E includes: a under clad 10; a circuit core 16, formed on under clad 10 and arranged from a main core 13 and two branch cores 14 and 15 that are connected to main core 13; and an over clad 11 that embeds circuit core 16. Main core 13 is connected to branch cores 14 and 15 at connection part 21 across a gap 18. Branch cores 14 and 15 are mutually spaced apart by a gap 17 at connection part 21. Also, branch cores 14 and 15 gradually separate from each other with distance from connection part 21.
In FIG. 14A, section line XIVB-XIVB′ indicates a position of an end face of main core 13 at the connection part 21 side, section line XIVC-XIVC′ indicates a position of end faces of branch cores 14 and 15 at the connection part 21 side, and section line XIVD-XIVD′ and section line XIVE-XIVE′ indicate positions of branch cores 14 and 15 away from connection part 21.
Guided light that is incident from an end of main core 13 that differs from the end at the connection part 21 side propagates through main core 13 and is distributed among branch cores 14 and 15 at connection part 21. By tandem connection of such Y branch circuits in a tree-like manner in N stages (where N is a positive integer), a 2N-branch circuit can be arranged.
The characteristics of a Y branch circuit are evaluated by excess loss L (dB) and branching ratio R (%). If the optical power of light propagating through main core 13 is P0, and the optical powers of light propagating through branch cores 14 and 15 are P1 and P2, respectively, the excess loss L (dB) and the branching ratio R (%) are respectively defined as follows:
                              L          ⁡                      (            dB            )                          =                              -            10                    ×                                    log              10                        ⁡                          (                                                                    P                    ⁢                                                                                  ⁢                    1                                    +                                      P                    ⁢                                                                                  ⁢                    2                                                                    P                  ⁢                                                                          ⁢                  0                                            )                                                          (        1        )                                                                    R              ⁡                              (                %                )                                      =                                                            P                  ⁢                                                                          ⁢                  1                                                                      P                    ⁢                                                                                  ⁢                    1                                    +                                      P                    ⁢                                                                                  ⁢                    2                                                              ×              100                                ,                                          ⁢          or                ⁢                                  ⁢                              R            ⁡                          (              %              )                                =                                                    P                ⁢                                                                  ⁢                2                                                              P                  ⁢                                                                          ⁢                  1                                +                                  P                  ⁢                                                                          ⁢                  2                                                      ×            100                                              (        2        )            In principle, R=50% can be realized by making the shape of the circuit axisymmetrical with respect to a central axis of main core 13. However, it is known that when branch core 14 or 15 deforms, the branching ratio deviates from 50% or becomes poor in reproducibility (see, for example, Japanese Patent Application Laid-open No. Hei 4-70605 (second column of p. 2 to third column of p. 3, FIGS. 1 and 4; referred to hereinafter as Patent Document 1), Japanese Patent Application Laid-open No. Hei 5-157925 (eleventh paragraph, FIG. 15; referred to hereinafter as Patent Document 2), and Japanese Patent Application Laid-open No. Hei 8-220359 (fifth paragraph, FIG. 5; referred to hereinafter as Patent Document 3)). Thus as shall be described later, gaps 17 and 18 are provided to prevent deformation of branch cores 14 and 15 (see, for example, Patent Document 1).
Branching of the guided light shall now be described in detail. The guided light that propagates through main core 13 propagates into branch cores 14 and 15 at connection part 21. Because branch cores 14 and 15 approach each other at connection part 21 (section line XIVC-XIVC′), the respective guided lights are coupled strongly. Guided lights of this coupled state are separated gradually into the respective branch cores while propagating through branch cores 14 and 15.
Preferably at connection part 21, the field distribution of the guided lights in the coupled state that propagate through branch cores 14 and 15 and the field distribution of the waveguided light at main core 13 are matched as much as possible. This is because mismatch of the two field distributions becomes the excess loss of the Y branch circuit. That is, ideally, gaps 17 and 18 are not provided. However as disclosed in Patent Document 1, it is generally preferable to provide gaps 17 and 18 to prevent fusion of branch cores 14 and 15 and forming of air gaps between cores 14 and 15.
A general method for manufacturing the Y branch circuit shall now be described with reference to FIGS. 15A to 15D. FIGS. 15A to 15D show end sections at the connection part 21 side of branch cores 14 and 15, and FIG. 15D corresponds to FIG. 14C.
First, as shown in FIG. 15A, core layer 12 is formed from a silica-based glass on a silica glass substrate that is to become under clad 10. It is known that this silica-based glass layer can be formed by the FHD (Flame Hydrolysis Deposition) method (see, for example, Japanese Patent Application Laid-open No. Sho 58-105111 (referred to hereinafter as Patent Document 4)). That is, by combusting a raw material gas having SiCl4 as a main component in an oxyhydrogen atmosphere, microparticles of silica-based glass are made to deposit onto under clad 10. By treating (sintering) the microparticles at a high temperature no less than the softening temperature of the microparticles, a transparent silica-based glass layer is obtained. The softening temperature and the refractive index of the silica-based glass are adjusted by mixing a gas of a chloride of germanium (Ge), boron (B), phosphorus (P), etc., in the raw material gas. Ge and P increase the refractive index, and B decreases the refractive index. All of Ge, B, and P lower the softening temperature of the silica-based glass.
To provide a waveguide structure, the refractive index of core layer 12 is set to be 0.2 to 5% higher than the refractive index of under clad 10. This value is called the relative refractive index difference and is calculated as follows:Relative refractive index difference (%)=(Refractive index of core layer−Refractive index of under clad)÷Refractive index of core layer×100The softening temperature of the silica-based glass of core layer 12 is set lower than the softening temperature of the silica glass of under clad 10 so that under clad 10 does not become deformed in the treating (heating) step for forming the glass.
Then etching masks 31, onto which circuit shapes have been transferred by a photolithography process, are formed on core layer 12 as shown in FIG. 15B. There is a limit to the resolution of etching masks 31, and it is generally difficult to form a fine pattern of no more than 1 μm with good reproducibility.
Thereafter, using etching masks 31 as protective layers, core layer 12 is removed by etching at parts besides the circuit core to thereby form core ridges 34 and 35 as shown in FIG. 15C. In this process, each core ridge is etched in lateral directions (horizontal directions with respect to the deposition surface of under clad 10) and the width thereof thus becomes approximately 1.5 μm narrower than the width of etching mask 31 (side etching). An interval D2 between two core ridges 34 and 35 is thereby made approximately 1.5 μm wider than an interval D1 of two etching masks 31.
Lastly, as shown in FIG. 15D, over clad 11 is formed from silica-based glass in the same manner as in the case of core layer 12. The refractive index of over clad 11 is set lower than that of core layer 12 and, if possible, is preferably made equal to the refractive index of under clad 10. Also to prevent deformation of core layer 12 (core ridges 34 and 35) in the heating treatment during glass formation, the softening temperature of over clad 11 is set lower then the softening temperature of the silica-based glass of core layer 12. A branch core interval D is substantially equal to the interval D2 between the two core ridges 34 and 35.
As mentioned above, to perform manufacture with good reproducibility, gap 17 is provided at connection part 21. However, a part of the guided light is dissipated at this part, giving rise to excess loss of the Y branch circuit. FIG. 16 shows simulation results of a relationship between the branch core interval D, which is the width of gap 17, and the excess loss of a Y branch circuit. The loss increases quadratically as the branch core interval D widens. With a normal manufacturing method, due to the resolution limit of the etching mask, the etching mask interval D1 must be made no less than 1 μm to form gap 17 with good reproducibility, and due to side etching during core processing, the gap between core ridges increases further by another 1.5 μm. That is, with the conventional technique, the branch core interval D is no less than 2.5 μm, and thus the excess loss of the Y branch circuit was 0.2 dB. For example, when five such Y branch circuits are connected in tandem to prepare a 32-branch circuit, the excess loss becomes 1 dB. Because the loss of an optical fiber is 0.2 dB per 1 km, an excess loss of 1 dB corresponds to decreasing the service range of FTTH by 5 km. The loss of a branch circuit is thus required to be lessened even by 0.1 dB.